Can anyone advise me is there any scripts in the Lifetime License that I can use to keep track of client subscription WHM/cPanel?

I understand WHMCS can accomplished this, but there is too many functions in it which I don't actually need.

All I need is to keep track of client subscription Start and End Date.

I am quite a newbie and some scripts are alien to me.
I found 2 free tools, The Hosting Tools and Free Hosting Manager but unsure how to integrate it into WHM/cPanel although they do provide a installation guide.

Sir you can install any of the 264 scripts (which includes The Hosting Tool) on your site. Just login to the Enduser Panel of Softaculous and search for the script you want to install and install it on your domain.
You may refer to the following guide on how to install a script:
http://softaculous.com/docs/Install_a_Script

If you do not find a script that you wish to install you can suggest the script we will review it and will add it if possible.